The invention relates to the fuel management of internal combustion engines. More particularly, the invention relates to the fuel supply systems of internal combustion engines in which the exhaust gas chemistry or composition is continuously monitored by at least one oxygen sensor whose signal is employed to engage a closed-loop control system for adjusting the fuel-air ratio of the combustible mixture supplied to the engine. The fuel supply system itself may be a carburetor or an electrical fuel injection system and the invention will be particularly useful in engines which employ a catalyzer for post combustion of fuel gases in the exhaust. The fuel control system to which this invention particularly relates would normally include a comparator that serves to compare the output signal from the oxygen sensor with a locally generated setpoint or reference value and the comparator output might be coupled into an integrating circuit for forming a fuel valve control signal, for example, or a signal used to adjust the fuel-air mixture in some other way. The fuel supply system may introduce the fuel continuously or intermittently into the induction manifold of the engine. When closed-loop control is used in this manner, the engine itself with its induction and exhaust manifolds becomes the controlled system while the fuel preparation system is the controller. It is well known that the oxygen sensor, also sometimes called .lambda.-sensor, which is normally located in the exhaust system of the engine, generates an abruptly changing output signal in dependence on the presence or absence of excess oxygen, i.e., depending on whether the original mixture fed to the engine is lean or rich. The comparator circuit then performs an electrical comparison of the output sensor voltage with a threshold or set-point signal and alters its own output in dependence on which of the two signals is smaller. The known control system thus permits a very exact continuous adjustment of the mixture to an air factor .lambda.=1, i.e. a stoichiometric mixture. One disadvantage of the aforementioned system is that the air factor .lambda. based on the set-point value supplied to the comparator will remain constant for all operational states of the engine. It is a demonstrated fact, however, that the chemical composition of the exhaust is a function of engine load for example, so that it might be advantageous to be able to shift operation of the engine to an air factor .lambda. which is variable within a very narrow range depending on engine load, for example. This is especially desirable if the engine is equipped with a catalyzer which itself has a very steep characteristic curve and must thus be operated at the precise stoichiometric point. However, the characteristic curve of the catalyzer can also be subject to shifts due to engine load which are then accounted for by the change in the set-point value fed to the comparator.